Bouyancy Mystifies Me

[tarponchik]

Definitely a matter of stability. I suspect a log could be made to float vertically, on a perfectly calm day, etc. Just like you could carefully balance the log on end on flat ground. But it would be very unstable with respect to any slight horizontal force and would flip over to horizontal (a position of lower potential energy).

You are almost there. But why is the horizontal position of the lowest potential energy?

 

[dead dog]

If the cube is made of the same material, there is no ‘heavier/denser’ part.

I think it is more about which position is more stable. In theory I think there are many positions which would be in equilibrium but in practice I think face up is more likely and less prone to be disturbed.

It's a matter of luck.

 

[BlueTrin]

It's a matter of luck.

It’s a theoretical problem

 

[BlueTrin]

You are almost there. But why is the horizontal position of the lowest potential energy?

I didn’t compute any maths but if you change the position to vertex yo for example would it being the center of gravity to a higher position ?

therefore it would require more energy to move it there from face up

um not too sure about what I typed. Seems wrong using symmetry, except if the half in the water is.

is there any surface tension in the solution ?

 

[ScubaRob0311]

Wouldn't a wooden cube float with one point down and one point up?

It would seem that a log and most things that float will do so with the most surface area in contact with the water.

 

[Jcp2]

I disavow my first link and stand by my second link.

 

[Couv]

 

[dberry]

You are almost there. But why is the horizontal position of the lowest potential energy?

If the log is positively buoyant, the center of gravity in the vertical orientation would be higher than when the log is horizontal. Lowest potential energy would be when the center of gravity is as low as possible. Think of an overloaded container ship with too many containers stacked too high. It might be floating (buoyant), but it would want to heel over.

That can't be the whole story, tho'. I suppose in the vertical position you've got more of the log submerged, thus buoyant forces are pushing more of the log up. Parts of the log out of the water are only subject to the force of gravity pulling down, without any buoyant force. The most stable position would have as little of the log submerged as possible.

I don't feel like thinking about this in any great detail, but I suspect exact calculations on the vertical log would require integration of the forces working on the submerged parts of the log (buoyancy and gravity) versus the part of the vertical log sticking out of the water. Physics has never been my strong suit, but this all brings to mind long-forgotten terms such as torque, angular momentum, and formulas for all sorts of rotating objects (and lever arms).

 

[BlueTrin]

I think one thing I didn’t realise is that, maybe it is not obvious the ratio between the submerged and out of water depending of the density of the wood.

 

[johndiver999]

If the log is positively buoyant, ............. I suppose in the vertical position you've got more of the log submerged, thus buoyant forces are pushing more of the log up.

I don't think position (orientation) of a homogeneous object is going to affect the density, the buoyancy or the portion of the object that is held above the water surface.